1. Field of the Invention
The present invention relates to a die used to mold a cage out of a synthetic resin and, more particularly, to a die used to mold a cage that holds balls of a ball bearing or sprags of a one-way clutch and to a method of molding such a cage, using the die.
2. Description of the Related Art
Generally, a cage made of a synthetic resin comprises a cylindrical body 10 and plural pockets 11 formed in the outer surface of the body, as shown in FIG. 6. The pockets 11 act to receive rolling balls or sprags and are circumferentially spaced from each other. The illustrated cage is a drilled cage. Besides, so-called crown-type cages (not shown) are available.
Such a cage made of a synthetic resin is normally fabricated by injection molding. This is now described in detail by referring to FIG. 7. An annular cavity 12 conforming to the profile of a molded cage is formed in a die. A molten synthetic resin is poured into the cavity 12 from a gate 13. The poured synthetic resin separates into two streams flowing to the left and to the right, respectively, inside the cavity 12. These two streams meet at the opposite side of the gate 13 and recombine into one stream.
If the resinous material poured from the gate 13 as described above moves a relatively long distance until the two streams meet at the opposite side of the gate 13, the molten resinous material cools down as the two streams go toward the meeting point. If the two cooled streams meet, a weld W weaker than other portions is formed at the meeting point. In the cage of the synthetic resin including this weld, it tends to incur breakage in use. Therefore, if the meeting point of the two streams of the poured resinous material is designed to form a thin-walled portion, the decreased cross section is combined with the weld, thus resulting in a still lower strength.
To compensate for the decrease in strength as described above, reinforcing materials such as glass fibers, carbon fibers, or metal fibers may be added. However, the added fibers disturb the orientation of the fibers at the meeting portion W of the two streams in the cage of synthetic resin. Again, the strength tends to decrease at this meeting portion W.
One method free of this drawback is to provide plural gates circumferentially spaced around the cavity. Streams of the synthetic resin poured from the gates move a short distance to the meeting points. Thus, the streams of the poured material meet such that the temperature is hardly lowered. This increases the strength of the meeting points.
However, as the number of the gates increases, the number of welds increases accordingly. Furthermore, the structure of the molding die is complicated. This complicated die is cumbersome to machine. Especially, where the molding die is designed to offer numerous molded products, gates must be increased for each of numerous cavities. Consequently, it is difficult to design the molding die itself. In addition, it is very difficult to machine the die. This leads to a great increase in the cost. For this reason, it is difficult to increase the number of gates. Moreover, the weight of the sprue runner increases. Additionally, a limitation is imposed on the maximum number of gates.
Where the cage has a large diameter, if the number of gates is increased, the gates may be more widely spaced from each other circumferentially. In this case, the poured resinous material flows a relatively long distance. Again, there arises the possibility that welds which might have insufficient strength are formed.